The present invention broadly relates to a new and improved construction of a ledge or ledge arrangement for a sheet forming zone of a double wire papermaking machine. The present invention further relates to a new and improved construction of a pressing unit for a double wire former of a papermaking machine.
In its more particular aspects, the present invention specifically relates to a new and improved construction of a ledge for a sheet forming zone of a double wire papermaking machine and which ledge is arranged substantially transverse to the machine direction in a substantially flexure resistant manner. The ledge is adjustable for guiding the sieves or screens or wires between which a sheet is formed from a web of material. A guide surface facing the wires, is provided at the ledge.
During the manufacture of paper or cardboard the stock suspension which is infed between the sieves or screens or wires of the papermaking machine, is subjected to pressures which vary in the machine direction. The aqueous portion of the stock suspension is withdrawn through the sieves or screens or wires and a sheet is formed therebetween. The pressure variations are necessary in order to prevent flocculation of the fibers. The pressure variations are favorable for accomplishing satisfactory sheet formation.
For this purpose, there are frequently provided pressing units above and below the sieves or screens or wires and such pressing units are equipped, as viewed in the machine direction, with a series arrangement of a predeterminate number of ledges disposed substantially transverse to the machine direction. Constructions of this type are known from, for example, German Patent No. 3,153,305, published Oct. 23, 1986, and cognate with U.S. Pat. No. 4,447,295, granted May 8, 1984, German Patent No. 3,503,242, published June 16, 1988, European Published Patent Application No. 0,160,615, published Nov. 6, 1985, German Published Patent Publication No. 3,815,316, published Nov. 16, 1989, and German Published Patent Application No. 3,830,683, published May 11, 1989, which is cognate with the cross-referenced U.S. application Ser. No. 07/391,573, filed June 26, 1989.
In the hitherto known constructions, an abrupt and high pressure surge occurs at each one of the ledges so that, in extreme conditions, there can be destroyed the sheet or web which is already formed between the sieves or screens or wires, or the formed web or sheet can be crushed. Particularly, wide-ranging irregularities in the weight per unit area of the web or sheet or the presence of fiber flocks therein may result in local crushing due to an excessively steep pressure gradient in the forming region. Formation of the web or sheet is thereby impaired and also the strength values or data of the web or sheet decisively deteriorate under such conditions.
Developments hitherto undertaken aim at reducing such problems during use of the ledges. Therefore, there have been suggested ledges having appropriately formed contours of the guide surface which faces the sieves or screens or wires whereby there is intended a small pressure surge at the infeed into the region of the ledge. However, a desirable success in this respect cannot be achieved solely through this measure. Furthermore, the hitherto proposed measures for adjusting the ledges relative to the sieves or screens or wires impede the effects which can be expected as a result of appropriate shaping of the guide surface because, due to the adjustment, the spacings between the sieves or screens or wires and the guide surface are eventually substantially changed along the contour of the guide surface.